Neural pathways of reflex-evoked behaviors and command systems in the abdomen of the crayfish

Abstract

1. The lifting of a crayfish (Procambarus)clarkii) from the substrate results in reflexive abdominal extension often accompanied by metachronous swimmeret beating. Cinematographic analyses show that the maintained, maximally extended abdominal position is stereotypic from trial to trial for each animal. 2. Complete transections at various levels of the abdominal nerve cord abolish the reflex-evoked extension and swimmeret behavior in all ganglia caudal to the lesion (Figs. 1, 2). Therefore, some essential portion of the pathways mediating these reflexive behaviors originates rostral to the abdomen and projects caudally. 3. Transection of any one of the paired abdominal connectives (i.e., hemiconnectives), or transection of up to 4 unilateral hemiconnectives has no effect on the reflex-evoked behaviors. 4. Transection of two hemiconnectives, done on opposite sides in adjacent abdominal segments, eliminates reflexive behavior in all ganglia caudal to the more posterior cut (Fig. 3). Hence, there is no evidence that the extension drive in one hemiconnective can cross within abdominal ganglia and travel in the opposite hemiconnective. 5. Similar transections of hemiconnectives in adjacent segments reveal that the swimmeret drive can cross ganglion 2 and course in the opposite hemiconnective (Fig. 3). No such ability was evident in ganglion 4 (Fig. 3). 6. Chronic recordings from postural motoneurons were also made during reflex-evoked behaviors (Figs. 4, 5). Compared to the intact animal, ligation of either hemiconnective in the segment anterior to the recorded ganglionic root reduces by approximately 50% the activity level of the extension motor program, even though the degree of movement remains the same (Figs. 4, 5). 7. A map of abdominal extension-evoking interneurons was made in cross-section of a thoraco-abdominal hemiconnective. Over 83% of these interneurons were found to occupy six loci (Fig. 6). 8. At least three of the locus-specific units have uninterrupted axonal projections through the abdominal cord (Figs. 7, 8). 9. No excitatory interactions between nonhomologous interneurons on one side of the nervous system or between bilateral homologous units were evident (Figs. 8, 9). 10. These characteristics of extension-evoking interneurons correspond closely to those of reflex-evoked extension drives, suggesting that their activity underlies reflexive abdominal extension behavior.